Breaking a leg only fits horror if #1 It was broken by a crazy fan who tied you to a bed, placed a block of wood between your legs, and pulled out a sledgehammer (gratuitous homage to King).#2 You broke it while running away from a serial killer/vampire/zombie/werewolf/or that one guy from 9th grade with the grabby hands#3 It was broken by a marrow sucking parasite who needs your tissue to survive. Of course, the tissue must be consumed while you are still living in order for the regenerative powers to take effect.